physics debug view fix (co-existing views in transitions)

Now, we no longer have citrus engine remove the debug view by name but instead it removes itself
when the StarlingPhysicsDebugView wrapper or the SpritePhysicsDebugView wrapper gets removed or disposed by StarlingArt or SpriteArt.

debug views are now in theory treated like any other kinds of view (there is probably room for improvement in their implementation too?) and so two can co-exist correctly on state transition.